Rotaviruses (RV) are the most important cause of severe diarrhea in young children worldwide. These viruses also cause diarrheal disease in healthy adults, the elderly, and the immune compromised. RV replicates primarily in the mature villous tip cells of the small intestine. Rotaviruses infect most mammals in a host-specific fashion; in general, RV strains that cause infection in one mammal do not cause disease in another species. The existence of several homologous murine RV strains that are natural pathogens of mice make the mouse model a highly tractable experimental system to study microbial host range restriction, pathogenesis, and immunity, especially as these factors relate to mucosal infections. Recent studies demonstrate that inhibition of innate immunity in a species-specific manner is a critical determinant of host range restriction. Natural RV infection effectively induces heterotypic (as well as homotypic) protection from symptomatic re-infection despite the existence of great serotype diversity among circulating RV strains. B cells and RV-specific antibodies mediate immunity to symptomatic re-infection. Neither the mechanism that results in broadly cross-reactive immunity nor the mechanistic basis for host range restriction is known. To address these two unknowns, we propose two Specific Aims: 1) Characterize in vivo the effects of homologous murine and heterologous simian RV infection on the STAT1 and STAT3 transcription factors to further unravel the mechanisms underlying RV host range restriction and innate immunity. Our hypothesis is that homologous RVs replicate successfully in the gut due to their ability to inhibit STAT1-dependent innate antiviral responses, and this is achieved by viral activation of STAT3. We propose to characterize STAT1/3- dependent innate signaling responses to infection at the protein and transcript level in individual intestinal cells using novl mass cytometry and microfluidic qRT-PCR technologies. 2) Determine, at the clonal immunoglobulin (Ig) level, the molecular basis for heterotypic protective immunity following RV infection. We will use a novel mouse model system ideally suited for the study of the clonal B cell response. Heterotypic (serotype cross-reactive) immunity plays a critical role in preventing RV disease and also is a highly desirable feature of several established (e.g., live attenuated influenza vaccine) and experimental (e.g., HIV and HCV) vaccines. We will define, at the level of individual Ig molecules, the nature and conditions for induction of heterotypic neutralizing reactivity. Our hypothesis is that serotypically diverse RV strains can be neutralized by individual lg molecules directed at either of the two serotype-specific RV surface proteins. To test this hypothesis, we will use a novel system to rapidly clone and express hundreds to thousands of RV-specific functionally active monoclonal antibodies (mAbs) from immunized mice. We hypothesize that individual mAbs with heterotypic specificity (as well with homotypic reactivity) are induced following a single natural antigen exposure and that these broadly reactive Ig molecules are responsible for the development of cross-protective immunity.